The Trinity in the Qur'an

A recent mail to Kevin
I do admire your staunch defence...
You say the plagiarism theory raises more problems than it solves since we must ask why Muhammad should have accepted some ideas and rejected others.
How should I know? There is no evident logic, despite your assertion of “internal consistency”, to what the author of the Qur’an chose from the Judeo-Christian mythology and what he rejected. Indeed, most of his decisions show a startling and (if I were a Christian or Jew) frankly insultingly simplistic understanding of his source material.
How, for example, do you explain Muhammad’s apparent belief that the Trinity means Christians worship three Gods: Jesus, Mary and God? (They are unbelievers who say, 'God is the Third of Three (thalithu thalathatin). No god is there but One God. 5:72 ..his mother was a just woman; they both ate food (er...I don't think anyone believes Mary didn't eat food! Not unless you mistakenly think that Mary was a god for some people...). 5:75 ...and when God said, ‘O Jesus son of Mary, didst thou say unto men, "Take me and my mother as gods, apart from God"?’ 5:116) If for no other reason, you surely must begin to wonder when the author of the Qur’an shows such a crass misunderstanding of the beliefs of the people of the book. This is not simply a matter of rejecting an idea but of ignorance. And how can God be ignorant?
(In fact, according to the earliest Muslim biographer ,Ibn Ishaq , the Quran was specifically correcting this supposedly erroneous belief in God, Mary and Jesus as three separate divinities. In his work, Sirat Rasulullah, Ishaq states that a Christian deputation from Najran came to debate Muhammad on the person of Jesus. Accordingly, these Christians allegedly believed that Jesus, "is God; and He is the son of God; and He is the third Person of the Trinity, which is the doctrine of Christianity...They argue that he is the third of three in that God says: We have done, We have commanded, We have created and We have decreed, and they say, If He were one He would have said I have done, I have created, and so on, but He is He and Jesus and Mary. Concerning all these assertions the Quran came down." ." (The Life of Muhammad: A Translation of Ishaq’s Sirat Rasul Allah, [Oxford University Press, Karachi, tenth impression 1995], p. 271))
Perhaps you will argue that God was simply correcting a heresy common at the time. It seems strange, then, to make the correction of a minor and long since forgotten heresy such a pillar of faith and such a proud boast in the final revelation to mankind.